Technical Field
The present invention relates to lithography processes, and, more particularly, to source, mask and target optimization for lithographic processes.
Description of the Related Art
Typical lithographic optimization methods, such as, for example, optical proximity correction (OPC), must work with pre-distilled target requirements. However, designing target shapes is relatively difficult in accordance with these methods, as the target shape needs to be re-designed iteratively and manually in a process referred to as retargeting.
In practice, the lithographic transfer process and the physics of the devices in the circuit both contribute in an integral way to overall successful functionality. However, in accordance with conventional methods, these two aspects are addressed separately via manual iterations between different engineering teams. Moreover, because the teams work in separate frameworks, neither team can easily design a substantial improvement in the overall process. For example, instead of maximizing yield, circuit designers are, at best, only able to provide the lithography team with shapes that satisfy simplified printability rules. As a result, lithography engineers are, at best, only able to identify small adjustments in target dimensions that are consistent with broad ground rules established by designers. These limitations arise even where yield of the most critical circuit structures are concerned, and the difficulty is compounded when the enormous volume of patterns present in a full layout is considered. OPC is inherently limited to simple considerations of edge placement error (EPE) only, thus, as a result, litho practice has typically segregated direct design considerations from automated mask data preparation.